Cobalt/magnesium transport protein, CorA. The structure at 2.7 Å resolution is known. The CorA monomer has a C-terminal membrane domain containing two transmembrane segments and a large N-terminal cytoplasmic soluble domain. In the membrane, CorA forms a homopentamer shaped like a funnel which binds fully hydrated Mg2+ in the periplasm (Maguire 2006). A ring of positive charges are external to the ion-conduction pathway at the cytosolic membrane interface, and highly negatively charged helices in the cytosolic domain appear to interact with the ring of positive charge to facilitate Mg2+ entry. Mg2+ ions are present in the cytosolic domain that are well placed to control the interaction of the ring of positive charge and the negatively charged helices, and thus, control Mg2+ entry (Maguire 2006). Gating is achieved by helical rotation upon the binding of a metal ion
substrate to the regulatory binding sites. The preference for Co2+ over Mg2+ is determined by the presence of threonine side chains in the channel (Nordin et al. 2013).

MIP family protein of 366 aas. Mg2+,Co2+ and the CorA-specific inhibitor (Co(III) hexamine chloride) bind in the loop at the same binding site. This site
includes the glutamic acid residue from the conserved "MPEL" motif (Hu et al. 2009).

Zn2+/Cd2+ efflux system, ZntB. Mg2+ is not transported. Wan et al. 2011 reported crystal structures in dimeric and physiologically relevant homopentameric forms at
2.3 Å and 3.1 Å resolutions, respectively. The funnel-like structure is
similar to that of the homologous Thermotoga maritima CorA Mg2+ channel and a Vibrio parahaemolyticusZntB (VpZntB). However, the central α7 helix
forming the inner wall of the StZntB funnel is oriented perpendicular to
the membrane instead of the marked angle seen in CorA or VpZntB.
Consequently, the StZntB funnel pore is cylindrical, not tapered, which
may represent an "open" form of the ZntB soluble domain. There are three Zn2+ binding sites in
the full-length ZntB, two of which could be involved in Zn2+ transport.